Traveling wave tube



June 4, 1963 H. A. HAUS ETAL TRAVELING WAVE TUBE Filed OCI'.. 22. 1959 205mm OPOmljO Q24 MQOIEQQ /NvE/vrons HERMA/v A. HAI/s JAMES A. MULLE/v Afro/mfr Patented June 4, l 1 963 3,092,750 TRAVELING WAVE TUBE Hermann A. Haus and James A. Mullen, Milton, Mass., assignors to Raytheon Company, Lexington, Mass., a corporation of Delaware Filed Oct. 22, 1959, Ser. No. 848,071 4 Claims. (Cl. S15-3.6)

This invention relates generally to traveling wave tubes and, more particularly, to means for reducing noise Within said traveling wave tubes.

In traveling wave tubes, it is well known that noise generated in the tube may be excessive for some applications, especially when the tube is used in receiver operation with low signal inputs. -Noise in traveling wave tubes generally arises because electrons are emitted from the electron gun at random velocities and at random times. Such random emission gives rise to random currents and voltages in the tube.

It has been conventional in prior art systems to reduce this noise by providing a series of auxiliary electrodes in the region following the electron gun. These electrodes provide additional accelerative forces for the electron beam as it travels down the tube. The auxiliary electrodes may be arranged to provide discreet jumps of acceleration or a substantially continuous acceleration of the beam before it moves along the path adjacent the delay line. Although such a structure reduces noise eects in the tube, the use of such accelerating regions is undesirable because the beam becomes defocussed and additional focusing elds are required to cause the beam to travel down through the tube in the correct path.

This invention, however, provides a system which reduces the noise to a minimum without the requirement of additional focusing fields to obtain correct beam travel. The invention makes use of the fact that two space charge waves exist in the electron beam, one of which travels faster than the electrons and the other of which travels more slowly than the electrons. The noise which accompanies the fast space charge wave is, in general, partially correlated with the noise associated with the slow space charge wave. The tube gain is obtained by the interaction of the R-F signal and the slow space charge beam wave.

In one embodiment of the invention wherein a traveling wave tube utilizes a helical delay line structure for providing interaction between lan electromagnetic wave, or Alll-F input signal, and the beam, the helical structure is divided into two sections connected together. The first helical section is arranged to provide interaction between the R-F signal and tbe fast space charge beam wave. The second helical section is arranged to provide interaction between the R-F signal and the slow space charge beam wave. In this way, a portion of the fast space charge power is coupled to the rst helical section. Noise associated with the fast space charge wave is, thus, transferred to the `first helical section and is propagated with the R-F signal along the helical structure. Since the noise in the fast and slow space charge waves is partially correlated, the noise due to the fast space charge wave partially cancels the noise transferred to the R-F signal due to interaction with the slow space charge -wave in the second helical section. Some uncorrelated noise is also present, but the over-all noise reduction is effective enough to provide theoretically the minimum noise of which the system is capable. Although some of the input power from the R-F signal is transferred to the fast space charge wave and lost as far as the output circuit is concerned, it is possible to make up this power by extending the length of the second helical section and, thus, increase the gain due to the slow space charge wave interaction.

The invention may be best described with the help of the accompanying FIGURE in which there is shown a schematic diagram of a traveling wave tube utilizing a particular embodiment of the structure of the invention.

In the figure, there is shown a traveling wave tube 10 having an electron gun system 11 which includes a cathode 12 and an accelerating beam forming anode 13. The beam as shown in this iigure travels down the tube substantially in a straight line as shown by dashed line 20. The traveling wave tube further includes a first helical section 14 and a second helical section 15, the latter including an attenuation means 16 the function of which is explained subsequently. It is advantageous to make the diameters of the first and second helical sections substantially equal, as shown in the iigure, although it is understood that such a construction is not necessarily required. There is also included in the tube a collector anode 17. The cathode and anodes lare connected to appropriate power supplies shown in the figure by block 18. The R-F signal from input source 21 is fed to one end of rst helical section 14 and R-F output is taken from one end of helical section 15.

`First helical section 14 is arranged so that its pitch, defined as the reciprocal of the number of turns per unit length, is such as to cause the R-F input signal to interact with the electron beam to cause an interchange of energy between the R-F signal and the fast space charge beam wave associated with the electron beam. In this Way, a portion of the noise associated with the fast space charge wave of the electron beam is coupled to the iirst helical section. The pitch of the helix of second helical section 1S is arranged so as to provide -an interaction between the R-F signal and the slow space charge Wave associated with the electron beam. Similarly, a portion of the noise associated with the slow space charge wave is coupled to the second helical section. Since there is a correlation between the noise associated with the fast space charge wave and the noise associated with the slow space charge wave, there is a substantial cancellation of the latter noise by the former noise. To achieve maximum noise reduction, the length of first helical section 14 is adjusted until a minimum noise can be measured on the R-F output signal removed at the output end of second helical section Y15.

A conventional attenuation means 16 is placed in slow wave helical structure 15 to suppress backward wave reflections so that the possibility of backward wave oscillations is eliminated.

Although one speciiic embodiment of the invention utilizes an adjustment of pitch and length to obtain interaction between the electromagnetic wave and the fast space charge wave in helical structure 14, it is obvious that any other means which provides interaction with the fast space charge wave vfollowed by subsequent interaction with the slow space charge wave would bring about the same noise reduction effect. Such a system essentially requires an adjustment of the well-known Pierces b parameter. The fast space charge wave is a wave which travels faster than the speed of the electrons and the slow space charge wave is one which travels more slowly than the electrons. When Pierces parameter b equals 0, the R-F signal phase velocity substantially equals the velocity of the electrons in the beam. Matching of the R-F signal phase velocity with the fast space charge wave requires adjustment of Pierces parameter b so that b is positive. Similarly, in order to match the slow space ycharge wave, Pierces parameter b is made negative. Such an adjustment of this parameter in each helical section may be brought about in any convenient manner. The adjustment is shown in the gure as accomplished by appropriately adjusting the pitch of the helices in each helical section. n

Although the particular traveling wave tube shown in the specific embodiment of the invention `described above utilizes helical :delay linestructures, it is not to 'be construed as the only embodiment of the invention. Any type of traveling wave tube delay line structure may be utilized, as long as the delay line structure `is divided into two portions, a first one having the R-F input signal matched to the fast space charge wave and the second having the R-F signal matched to the slow space charge wave. Hence, the invention is not tofbe construed as limited to the specific embodiments shown and described herein except as Idefined by the appended claims.

What is claimed is:

l. A traveling wave tube comprising an electron source; means for directing a beam of electrons from said source along an extended path, said electron beam having a first space charge wave traveling Iat a rate faster than said electrons and a second space charge wave traveling at a rate more slowly thanV said electrons; lhelical delay line means being positioned adjacent said extended path and including rst and second interconnected helical delay line sections; means for propagating an electromagnetic wave along said first and said second helical delay line sections; the pitch of said first helical delay line section being selected to provide an interaction -between said electromagnetic wave and said first space charge wave as said beam travels along a path adjacent said first helical delay line section and the pitch of said second helical delay line section being selected to provide an interaction 4between said electromagnetic wave and said second space charge wave as said beam travels along a path adjacent said second helical `delay line section, said electromagnetic wave being thereby amplified; output coupling means for removing said amplified electromagnetic wave from said traveling wave tube.

2. A traveling wave tube comprising an electron source; means for directing a beam of electrons from said source along an extended path, `said electrom beam having a first space `charge wave traveling at a rate faster than said electrons and a second space charge ywave traveling at a rate more slowly than said electrons; helical delay line means being positioned adjacent said extended path and including first and second interconnectedV helical delay line sections; means for propagating an electromagnetic wave along said first and said second helical delay line sections, the pitch of said first helical delay line section being selected to provide an interaction between said electromagnetic wave and said first space charge -wave as said beam travels along a path adjacent said first helical `delay vline section and the pitch of said second helical delay line section being selected to provide an interaction between said electromagnetic Wave and said second space charge wave as said beam travels along a path adjacent said second helical delay line section, the pitch of said first helical delay line section being less than that of said second helical delay line section; output coupling means for removing said electromagnetic wave from said traveling wave tube.

3. A -traveling wave tube comprising an electron source; means for directing a 4beam of electrons from said source along an extended path, said electron beam having a first space charge wave traveling at a velocity greater than that of said electrons and a second space charge wave traveling at a velocity less than that Vof said electrons; helical delay line means being positioned adjacent said extended path and including first and second interconnected helical delay line sections, said second helical section including attenuation means for preventing backward wave oscillations within said traveling wave tube; means for propagating an electromagnetic wave along said first and second helical sections; the pitch of said first helical section being selected to provide an interaction between said electromagnetic wave and said rst space charge wave and the pitch of said second helical section being selected to provide an interaction between said electromagnetic wave and said second space charge wave; output coupling means connected to said second helical section for remov- Ying said electromagnetic wave from said traveling wave Y oscillations within said traveling wave tube; means for ropagating an electromagnetic wave along said first and second helical sections, the pitch of said first helical section being selected to provide an interaction between said electromagnetic wave and said first space charge wave and the pitch of said second helical section being selected to provide an interaction between said electromagnetic wave and sm'd second space charge wave, the length of said first helical section being adjusted so as to reduce the noise associated with said electromagnetic wave to a minimum; output coupling means connected to said second helical ection for removing said electromagnetic wave from said traveling wave tube.

References 'Cited in the file of this patent vUNITED STATES PATENTS 2,584,597 Landauer Peb. 5, 1952 2,767,259 Peter Oct. 16, 1956 2,908,844 y'Quate Oct. 13, 1959 3,009,078 Ashkin Nov.Y 14, 41961 FOREIGN PATENTS 696,058 -Great Britain Aug. 26, 1953 1,041,169

Germany Oct. 16, 1958 

4. A TRAVELING WAVE TUBE COMPRISING AN ELECTRON SOURCE; MEANS FOR DIRECTING A BEAM OF ELECTRONS FROM SAID SOURCE ALONG AN EXTENDED PATH, SAID ELECTRON BEAM HAVING A FIRST SPACE CHARGE WAVE TRAVELING AT A VELOCITY GREATER THAN THAT OF SAID ELECTRONS AND A SECOND SPACE CHARGE WAVE TRAVELING AT A VELOCITY LESS THAN THAT OF SAID ELECTRONS; HELICAL DELAY LINE MEANS BEING POSITIONED ADJACENT SAID EXTENDED PATH AND INCLUDING FIRST AND SECOND INTERCONNECTED HELICAL DELAY LINE SECTIONS, SAID SECOND HELICAL SECTION INCLUDING ATTENUATION MEANS FOR PREVENTING BACKWARD WAVE OSCILLATIONS WITHIN SAID TRAVELING WAVE TUBE; MEANS FOR PROPAGATING AN ELECTROMAGNETIC WAVE ALONG SAID FIRST AND SECOND HELICAL SECTIONS, THE PITCH OF SAID FIRST HELICAL SECTION BEING SELECTED TO PROVIDE AN INTERACTION BETWEEN SAID ELECTROMAGNETIC WAVE AND SAID FIRST SPACE CHARGE WAVE AND THE PITCH OF SAID SECOND HELICAL SECTION BEING SELECTED TO PROVIDE AN INTERACTION BETWEEN SAID ELECTROMAGNETIC WAVE AND SAID SECOND SPACE CHARGE WAVE, THE LENGTH OF SAID FIRST HELICAL SECTION BEING ADJUSTED SO AS TO REDUCE THE NOISE ASSOCIATED WITH SAID ELECTROMAGNETIC WAVE TO A MINIMUM; OUTPUT COUPLING MEANS CONNECTED TO SAID SECOND HELICAL SECTION FOR REMOVING SAID ELECTROMAGNETIC WAVE FROM SAID TRAVELING WAVE TUBE. 